Digital indicator



Aug. 16, 1966 R. W. STUART, JR

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ROBERT W. STUAR'RJR.

BY Rama Sm ATTORNEYS Aug. 16, 1966 R. w. STUART, JR

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INVENTOR. ROBERT W. STUART,JR.

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ATTORNEYS Unite i 3,267,262 DllGlllAi, TNDICATOR Robert W. Stuart, .lr., Wobum, Mass, assignor to Computer Control Company, Incorporated, Framingham,

Mass., a corporation of Delaware Original application July 28, 1961, Ser. No. 127,690, new

Patent No. 3,109,923, dated Nov. 5, 1963. lDivided and this application Mar. 19, 1963, Ser. No. 266,279

11 Claims. (Cl. 235--92) This application is a division of my copending application Ser. No. 127,690, filed July 28, 1961, now Patent 3,109,928, granted Nov. 5, 1963.

My invention relates to digital indicators, and particularly to an improved code converting digital indicator having low voltage swing requirements for use with limited output devices such as transistorized binary counters and the like. 7

A necessary component of many electronic systems is a readout device that will display information visually, in digital form, in response to digital information generated at a random, and at times extremely rapid, rate. Usually, the codes best adapted for use in a particular electronic system are unsuitable for visual interpretation. Therefore, it is often desired that an indicator perform a code conversion prior to indication. For example, it may be desired to indicate in decimal form the number of pulses fed to a binary counter as determined by the binary output state of the counter. lit is frequently undesirable to use electromechanical indicators for this purpose, since the time delays inherent in such indicators place a limitation on the speed of response of the system in which they are used, which may defeat the purpose of the system. To avoid such delays, incandescent lamps or neon glow-tubes are commonly used as indicators in high speed systems.

Since the neon glow-tube is a bistable device, it can be used to perform logic functions, and is therefore preferable to the incandescent lamp for use in indicators requiring logical operations, such as code conversion, be tween the input and the output display. The relatively high voltage swing required to ionize and de-ionize commercial forms of neon glow-tubes has, in the past, him ited their practical application to circuits driven by vacuum tubes. In accordance with my invention, this difficulty is avoided by the use of a novel decoding matrix employed neon glow-tubes and resistors so arranged that a voltage swing smaller than the voltage normally required to ionize one of the "glow-tubes if sufiicient to change the state of the matrix.

An additional advantage of the digital indicator of my invention is that it is adapted for electronic blanking during counting, when, particularly on large panels employing many banks of indicators, the intermitted flashing of indicators can be extremely irritating and annoying to an operator or observer stationed at the panel.

Briefly, my invention comprises a digital indicator having a first set of neon glow-tubes arranged in an ordered array, each of which is assigned a decimal digit value, so that when a particular tube of the set is ionized and glowing, while the rest of the tubes are deionized, the digit associated with the glowing tube is indicated. Associated with each of the indicating tubes is one or more inhibiting glow-tubes having one terminal connected to a terminal of the indicating tube. The common terminals of the indicating and inhibiting tubes are connected to a source of voltage through a voltage dropping resistor. The opposite terminals of the indicating lamps are connected to the binary units output terminals of a binary counter, in a manner to be described in detail below, through one or the other of a first pair of resistors of a relatively high resistance value, and the opposite terminals of the inhibiting tubes are connected to selected 3,257,262 Patented August 16, 1966 terminals of the counter through resistors of relatively lower value. By this arrangement, conduction of an inhibitor tube so lowers the voltage at the common terminal of an associated indicator tube that it cannot be ionized, whereas conduction of an indicator tube does not lower the voltage at the common terminal of an associated inhibitor tube enough to prevent its ionizatirn. Thus, the inhibitor tubes are ionized preferentially, and control the ionization of the indicator tubes. An important advantage of this arrangement is that the voltage swing required to be applied to a given tube terminal need only be a relatively low value which is readily attained at the output terminals of a conventional transistorized counter.

My invention will best be understood by reference to the accompanying drawings, in which,

FIG. 1 is a schematic wiring diagram of an indicating system in accordance with one embodiment of my invention;

FIG. 2 is a schematic wiring diagram showing the details of a bistable circuit which may be employed as a component of the system shown in FIG. 1;

FIG. 3 is a schematic wiring diagram of a blanking circuit which may be employed in the system of FIG. 1; and

FIG. 4 is a charge showing the condition of the various indicating and inhibiting tubes employed in the system of FIG. 1 as a function of the conditions at the input terminals of the various units in FIG. 1 in the various stages of the counting cycle.

Various conventions have been employed in the drawings for the purpose of simplifying the description, and facilitating the understanding, of my invention, which will not necessarily have any significance in a physical embodiment thereof. Specifically, where convenient to do so, I have arbitrarily shown dotted lines surrounding a portion of the apparatus that is best understood as a unit, and where such units are repeated in the system I have merely shown them in block form without repeating their internal wiring. For reference purposes, I have arbitrarily shown terminals defining inputs and outputs of these blocks, and have assigned reference characters to these terminals. Of course, it will be readily apparent to those skilled in the art that these conventions are employed merely for the purpose of illustration, and that in practice the components of the circuits shown will be arranged as dictated by conventional wiring methods.

Referring now to FIG. 1, I have shown an input unit 1 which can be any desired computer or control unit, or a component thereof, which includes means for supplying a series of electrical pulses that it is desired to count between an output terminal a and a grounded terminal b. This unit may include a gate or other conventional switching device for switching the pulse supply means between active and inactive states, and means for producing a DC. voltage between an auxiliary output terminal c and ground terminal 11, when the unit is in its active state and emitting pulses, for purposes to be described. The pulses emitted by circuit 1 may be of any desired repetition rate, and the rate may vary randomly. In the illustrated embodiment, it is assumed that the pulses are positive-going, although negative-going pulses could be employed if so desired with obvious changes in the wiring to be described.

Pulses from unit 1 are supplied to input terminal a of a counter 2, which, as shown may be a conventional binary counter employing four bistable units F1, F2, F3 and F4, arranged in cascade and having feedback connections to provide 10 ldiscnete output states of the counter output terminals d. e, f, g, h, i, i and k in response to succeeding input pulses, such that the counter counts from 0 to 9, and then repeats.

designated as UA, UB, UC and UD.

1 sistor Tl will be held at The output terminals of counter 2 are connected to a digital indicator 3, to be described in detail below. Generally speaking, indicator 3 comprises 10 indicating neon glow tubes K through K9, and inhibiting neon glow-tubes 10 through 19. For purposes of illustration, the circuits interconnecting these tubes have been broken into groups to be considered as units, which units are In a manner to be described in detail below, these units are energized from the output terminals of counter 2 in such a manner that one and only one of the indicating tubes K0 through K9 is energized in each state of the counter. In practice, indicator tubes K0 through K9- would be mounted in a visible, ordered array, while tubes 10 through 19 would be concealed from view by a panel, cover or the like.

Also connected to indicator 3 is a blanking unit 4, which functions to extinguish all of the tubes in indicator 3, either automatically in response to a condition of the input unit, or as desired.

The power supply for the system of FIG. 1 may be of any desired conventional design, but as here schematically shown comprises a unit 5 incorporating batteries connected to supply a reference potential at a ground terminal G and three additional potentials B, N1 and N2 at the correspondingly designated terminals. For example, in one practical embodiment of my invention, the values of these potentials were 6 volts for terminal B, --l'50 volts for terminal N1, and -300 volts for terminal N2.

Before describing the system of FIG. 1 in further detail, the detailed structure and operation of certain of the components of the system will be described. First, referring now to FIG. 2, a typical bistable unit F1 is shown, which may be employed for use as any of bistable units Fl through F4 in FIG. 1. this regard that the arbitrary external terminal-s shown on the circuit of FIG. 2 correspond to similarly designated terminals of the bistable units shown in FIG. 1.

The circuit of FIG. 2 is essentially conventional in design and operation, and will be generally familiar to those skilled in the art. Therefore, it will be described only briefly. Basically, the circuit comprises two transistors T1 and T2, which are connected in a bistable circuit such that only one of the transistors is conducting at a time, and such that the circuit is stable with either transistor conducting. As shown, the transistors are of the p-n-p type, although n-p-n transistors are equally suitable with appropriate changes in the bias potentials. The emitters of transistors T1 and T2'are connected together and grounded at ground terminal g. The collectors are connected to symmetrical voltage dividers, comprising resistors R and R16 for transistor T1, and resistors R17 and R18. for transistorsTZ, which are connected between supply terminal c and ground terminal g as shown. The bias circuit is completed by connections from the bases of transistors T1 and T2, through resistors R14 and R13, respectively, to supply terminal 15 of the bistable unit Fl. Referring to FIG. 1, it is seen that supply terminal 0 is connected to terminal N1 of the power supply, which has a suitable negative potential, of, for example, l50 volts, and terminal b is connected to terminal B of the power supply, at a positive potential of for example, 6 volts. Thus, assuming that transistor T2'is conducting, its collector and output terminal a of unit F1 will be at ground potential, and the base of trana value sufficient to cut it off through the coupling network comprising resistor R19 and capacitor C4 connected between the collector of transistor T2 and the. base of transistor T1. With transistor T1 cut off, the coupling network comprising resistor R20 and capacitor C5, connecting its collector and the base of transistor T2 will assist in forward biasing the emitter of transistor T2 with respect to its base. In this state, the potential of output terminal a will be approxi- It should be noted in mate-1y N/: in the specific embodiment here described.

The state of the bistable unit just described may be considered the zero state as it is used in the system of FIG. 1. The unit may be returned to this state by conventional reset means, not shown, which may, for example, comprise means for opening the circuit between the emitter of transistor T1 and ground momentarily to force transistor T1 to cut otf. Since the bistable unit does not form a part of my invention, however, such details have not been shown.

The state of bistable unit F1 may be changed by a positive-going pulse applied to input terminal a. This pulse is applied through capacitor C3 and across resistor R12 to the junction of two diodes D3 and D4 which have their opposite terminals connected to the bases of transistors T2 and T1, respectively, as shown. Assuming that transistor T2 is conducting and transistor T1 is nonconducting, a positive pulse applied to the bases of the transistors will not effect transistor T], but will cut otlf transistor T2. The consequent drop in its collector potential will bias transistor T1 to conduction. The succeeding pulses applied to input terminal a will cause the unit to revert to its original state, in a manner that will be well understood by those skilled in the art.

An additional terminal f is shown on unit F1, which is connected to the base of transistor T2. This connection is for the purpose of applying a feedback pulse, in a manner to be described, which is positive-going, and which will cause transistor T2 to be driven to its nonconducting state. This action is necessary in the operation of the counter, as will appear.

Referring now to FIG. 1, it will be seen that the out put terminal e of each of the bistable units F1, F2 and F3 is connected to input terminal a of each of the succeeding bistable units. Thus, ignoring the efiect of the feedback network, each time one of the bistable units F1,

F2 and F3 is driven from its normal state, with a negative voltageappearing on its output terminal a and ground voltage appearing on its output terminal e, to the opposite state, a negative going pulse will be applied to the input terminal a of the succeeding bistable unit, which will not affect its operation. However, when one of these bistable units is driven back to its normal state, the terminal e going from a negative potential to ground potential, a positive-going pulse will be produced that will drive the following bistable unit to its opposite state. A binary sequence would thus be enforced in which unit F1 would change state on every pulse, unit F2 on every other pulse, unit F3 on every fourth pulse, and unit F4 on every eighth pulse. With the feedback connections, however, a different sequence is enforced. From output terminal d of unit F4, a feedback connection is made which is tied through capacitors C1 and C2, respectively, and diodes D1 and D2, respectively, to the feedback terminals 1 of bistable units F2 and F. Resistors R10 and R11, respectively, are connected between the junctions of the capacitors and diodes and ground, as shown. With this arrangement, the count proceeds normally up through 7, as described above, but at the count of 8 bistable unit F4 is reversed for the first time in the cycle, and a positivegoing pulse is applied from its output terminal d to input terminal 1 of units F2 and F3.

Refer-ring now to FIG. 4, in'conjunction with FIG. I, the effect of this feedback will be considered. First, in reading FIG. 4 it should be understood that the characters 0 and. 1 associated with the respective counter terminals at a particular count of the cycle are to be understood to mean that a voltage of ground potential exists on the tenminal where the symbol 0 appears, and the potential of N /z appears on the terminal where the sign 1 appears. Thus, on count 0 each of the terminals, 0, g, i and k of the counter, which are connected to terminals 0 of bistable units Fl through F4, are at 0 or ground potential, and each of terminals d, f, h and j of the counter,

corresponding to terminals a of the bistable units are at N /2 volts as indicated by the symbol 1 in FIG. 4.

At count 1, the first bistable unit F1 is reversed, and the other units remain the same. reversed again and unit F2 receives a positive pulse that causes it to reverse, with the remaining units unchanged. At count 3, only unit 1 is reversed. At count 4, unit 1 is reversed, causing unit 2 to reverse again, and this time a positive pulse is applied to unit F3 to reverse it. At count 5, again only unit 1 is reversed. At count 6, units 1 and 2 are reversed. At count 7, only unit 1 is reversed.

At the eighth count, unit F1 is reversed and applies a positive-going pulse to unit F2. Next, unit F2 will go over, causing unit F3 to go over, which in turn causes unit F4 to go over. When unit F4 does so, a positivegoing pulse is applied to the feedback terminals of units F2 and F3 which resets them to their original state. Thus, the net result of the eighth count is to reverse only units F1 and F4.

At the ninth count, unit F1 is reversed, and since a negative-going pulse is produced at its output terminal e, the remaining units are unchanged. The next, or tenth, pulse will cause unit-s F1, F2, F3 and F4 all to reverse. The feedback connections are ineffective at this time, since in this transition of F4 the feedback pulse is negative-going and is blocked by diodes D1 and D2. Thus, the counter is returned to the 0 count state on the tenth count, and thereby exhibits l0 and only discrete states which are cyclically encountered in response to succeeding input pulses applied to terminal a of the counter.

It will be apparent to those skilled in the art that either the sequence of states of terminals a, f, h and 1 or e, g, i and k of the counter, determine the count in terms of a particular binary code. With suitable revisions in the feedback connections, it is apparent that other codes could be formed if so desired.

Before describing the digital indicator 3 in detail, the details of blanking unit 4 will first be described Referring now to FIG. 3, in combination with FIG. 1, the blanking unit comprises a device for supplying either a constant output potential at its output terminal c of the potential N1, or a potential of lower magnitude, in a manner which can be controlled either manually or automatically. As will appear, terminal 0 of unit 4 is connected to supply the indicator and inhibitor tubes of indicator 3, and the lower negative potential at times supplied by unit 4 is insufiicient to ionize any of the tubes. Accordingly, the overall function of unit 4 is to extinguish all the tubes of indicator 3 at certain times.

Since a possible application of such a blanking unit is to prevent the tubes from flashing when the count is too rapid to be visually interpreted, I have shown a specific embodiment in which the blanking may be carried out automatically when the frequency, or rate of repetition, of the incoming pulses from source 1 is above a predetermined level. For this purpose, as shown in FIG 1, output terminal a of pulse source 1 is connected to input terminal a of blanking unit 4. Comparing FIGS. 1 and 3, it will be seen that terminal b of unit 4 is connected to terminal N1 of power supply 5, terminal c of unit 4 is connected to indicator 3 in a manner to be described, terminal d of unit 4 is connected to terminal B of power supply 5, terminal e of unit 4 is connected to terminal N2 of power supply 5, and terminal f of unit 4 is connected to ground.

Referring now to FIG. 3, input terminal a is connected to the input of a high-pass filter 6. Filter 6 may be designed in a conventional manner to cut off below any desired frequency above which it is undesired to operate the indicator, and above this frequency to apply an output voltage to a current amplifier and limiter 7 over a doublethrow switch 8 in a first position as shown. Amplifier 7 may be of any conventional design, it merely being required that it supply a constant current output in response to any appreciable input voltage from filter 6. In one At count 2, unit F1 is specific embodiment of my invention, the current supplied was 1.7 milliamperes for blanking, and O milliampere when the blanking circuit was inactive.

The filter 6 and amplifier 7 combine to produce a blanking signal, which may be fed to the blanking circuit proper in any suitable manner, and for example, as here shown, through a manual switch S which has two armatures a and b operable between an automatic position A and an off position 0 has shown. In the A position, the output of amplifier 7, or other suitable blanking signal source, is connected to the input of the blanking circuit proper, to be described. In the 0 position of switch S, terminal b of blanking unit 4 is connected to output terminal 0, thereby supplying potential N1 to terminal 0.

In many systems, it will be desirable to blank the indicator when the pulse source 1 is active and producing pulses at any frequency. For this purpose, comparing FIGS. 1 and 3, auxiliary terminal 0 of source 1, which is energized when the source is active, is connected directly to the input of amplifier 7 over terminal g of unit 4 and switch 8 in its other position.

The blanking circuit comprises a potential divider which may be modified by the condition of two transistors T3 and T4. A first potential is established at the junction of two resistors R21 and R24, which have their extremities connected between terminal e and ground terminal 1. It will be recalled that terminal e is at a potential of N2 volts, which could, for example, be 30O volts. The junction of resistors R21 and R24 is connected to the base of transistor T3. A second portion of the potential divider comprises resistors R22, R23, R25 and R26 connected between the terminal e and ground terminal ,1. As shown, output terminal 0 is connected to the junction of resistors R22 and R23. The collector of transistor T3 is connected to the junction of resistors R23 and R25, and its emitter is connected to the collector of transistor T4 and to the junction of resistors R25 and R26. The emitter of transistor T4 is connected to ground terminal 1. The base of transistor T4 is connected through a resistor R27 to supply terminal d, which is at potential B, which may, for example, be +6 volts.

In operation, with switch S in its A position and no current flowing in the output circuit of amplifier 7, both of transistors T3 and T4 are cut off and output terminal 0 is at a relatively high negative potential. However, when amplifier 7 is energized and current flows through its output circuit, it being connected such that the flow of current is away from terminal d, the base potential of transistor T4 is lowered below ground potential and transistor T4 conducts essentially to saturation. At the same time, the base of transistor T3 is maintained at a negative potential while its emitter is essentially raised to ground potential. Accordingly, transistor T3 also conducts essentially its saturation value of current, and the junction of resistors R23 and R25 is brought essentially to ground potential. Therefore, the potential at output terminal c is raised to a relatively small negative voltage which is insufficient to ionize any of the indicator tubes.

Referring again to FIG. 1, the detailed structure and operation of digital indicator 3 will next be described. As shown, indicator 3 may be considered to be functionally divided into three identical units UA, UB and U0, and a fourth unit UD.

Since units UA, UB and UC are identical, only unit UA will be described. It comprises two indicator tubes K0 and K1, two inhibitor tubes 10 and 11, and three resistors R0, R1 and RA. The indicator tubes are connected together at a common terminal. Resistor RA is connected between this common terminal and voltage supply terminal e, which is connected in turn to output terminal 0 of blanking unit 4. The other terminals of indicating tubes K0 and K1 are connected to input terminals a and b, respectively, or unit UA, which in turn are connected through resistors RE and RF, respectively, to output terminals 6 and d, respectively, of counter 2. The

other terminals of inhibitor tubes 10 and 11 are connected through resistors R and R1 to terminals d and 0, respectively, of unit UA. Resistors RA, RE and RF may be equal to each other and of a relatively high value. Resistors R0 and R1 may be equal to each other and of a value considerably smaller than resistors RA, RE and RF, for example, approximately one-third of the size of these resistors.

It is believed that the operation of the indicator as a whole will best be understood if the operation of unit UA with the associated resistors RE and RF is first considered independent of the other apparatus. The external conditions imposed are that terminal 0 of unit UA is energized with a potential of N1 volts. Terminals d and e of counter 2, connected to resistors RF and RE, respectively, are energized either with ground potential or with a voltage of approximately /2N1 volts, such that one or the other but not both of these terminals is at ground potential. In the operation of the indicator, as will appear, where more than one unit such as UA is employed the a terminals of all of the units are connected together, as are the b terminals. One of the indicator tubes in one and only one of the units so connected will always be ionized when the appropriate counter terminal d or e is grounded. Also, none of the indicator tubes connected to the counter terminal d or e which is energized with /2N1 volts will be ionized. Conduction through an ionized indicator tube will drop the potential at its associated terminal a or b to a value lower than ground, where N1 is negative; for example, the potential may drop to 45 volts. Thus, terminals a and b are energized either with /2Nl volts or with a voltage below ground equal to the drop across resistor RF or RE caused by the current fiow through one ionized indicator tube. Terminals c and d may each be at either ground potential or at N /2 vol-ts, independent of one another. With these conditions in mind, the various conditions shown in Table 1, below may occur. In Table 1, for later reference, all of the input states supplied by counter 2 are included. Since there are ten states of the counter and only eight discrete inputs to unit UA under the conditions set forth above, there are two states repeated in the table. Thus, unit UA is in the same state at counts 6 and 8, and again at counts 7 and 9. As in the table of FIG. 4, the 0s and 1s beneath the various terminals refer to ground and NV; potentials, respectively, except that for terminals a and b, the Os represent a negative voltage equal to the drop across RE or RF caused by the current through one indicator tube. In the columns under UA output, the states of the tubes in the unit are represented by 0 for an extinguished tube, which is not ionized, and 1 for an ionized tube Which is conducting and glowing.

Table I UA Terminals UA Output Count a b c (1 K0 K1 I 10 J 11 0 1 1 1 1 0 0 0 1 0 1 1 0 1 0 0 0 1 l 0 0 0 1 0 1 0 1 0 0 O 1 0 0 1 0 1 0 0 0 1 1 0 0 1 0 0 0 1 0 1 0 0 0 0 1 1 1 O 0 O 0 0 1 1 0 1 0 0 0 0 1 1 1 0 0 0 0 0 1 1 Referring to FIG. 1 in conjunction with Table I above, it will be seen that only one of the indicator tubes K0 and K1 can be energized at a time because only one of the terminals d and e of counter 2 can be at ground potential at a time. When either of these terminals is at ground potential, if the common terminal of the indicator tubes is at a potential of N1 volts, then the corresponding tube will be ionized. When an indicator tube is ionized, current flow through resistor RA raises the common potentialtoward ground. As to the other indicator tube, with the corresponding counter terminal at a potential of N /2 and the common terminal at a potential of less than N1, there is insuflicient voltage across it to cause ionization. Thus, a voltage swing of only a little more than one-half of that which is required to ionize the tube need be applied to the input terminals.

Inhibitor tubes 10 and 11 may be ionized regardless of the state of indicator terminals a and b, but neither of the indicator tubes can be ionized if either of the inhibitor terminals 0 and d is grounded. The reason for this is that due to the smaller value of resistors R0 and R1, conduction of tube 10 or 11 draws enough current to bring the potential of the common terminal too close to ground to permit ionization of the indicator tube by applying the intermediate voltage to its other terminal a or b. On the other hand, conduction of either of tubes K0 and K1 draws so little current through the larger resistor RE or RF that the potential at the common termi nal remains high enough to permit ionization of an inhibitor tube by grounding its terminal c or d. Thus, where there is a choice, the inhibitor tube is ionized in preference to the indicator tube and accordingly controls the indicator tube.

Each inhibitor tube can also inhibit an adjacent inhibitor tube. However, the smaller resistors in series with the inhibitor tubes take a smaller percentage of the voltage drop, and it is possible for two inhibitor tubes to be ionized in the same unit. Whether or not this will happen will depend on the exact voltages employed; in experiments with a physical embodiment of my invention, in which the power supply was energized from a 60 cycle, nominally volt line, I have found that under conditions of normal or relatively high voltage, both inhibitor tubes in a unit would ionize when called on to do so, whereas under low line voltage conditions only one of the inhibitor tubes would ionize. In the latter case, when both tubes were called on to ionize simultaneously, the predominating tube would be randomly selected. In cases, such as the transition between counts 5 and 6 shown in Table I above, in which an attempt was made to ionize the second tube with the first tube conducting, the second tube would not conduct under low voltage conditions. While of theoretical interest, this phenomenon is actually irrelevant to the operation of the indicator of my invention, because as long as one inhibitor tube in the unit is ionized, it makes no difference whether a second or third tube is ionized or not.

The structure and operation of units UB and UC are identical with those of unit UA. The various states assumed by these units in response to corresponding states of counter 2 are shown in FIG. 4. In FIG. 4, it should be recalled that the Os have a somewhat different significance as applied to terminals a and b of units UA, UB, UC and UD than they do as applied to the corresponding counter terminals 0. and e, because of the drop across either resistor REor resistor RF, as explained above.

The structure and operation of unit UD, as shown in FIG. 1, is similar in principle to the other units just described, except for diflerences in wiring to properly follow the logic of the last four counts. As shown, indicator tubes K6 and K7 form a pair having a common terminal, to which is connected one terminal of a supply resistor RDl and one terminal of each of three inhibitor tubes 16, 17 and 18. The other terminal of resistor RDl is connected to supply terminal 11, and thence to output c of blanking unit 4, to be energized at the potential N1 under normal conditions, and at a much lower voltage under blanking conditions, as described above. The other terminals of inhibitor tubes 16, 17 and 18 are connected to input terminals g, f and e, respectively. Resistor RD1 has the same value as resistor RA in unit UA, and resistors R6, R7 and R8 are equal in value to resistors R0 and R1 of unit UA.

Indicator tubes K8 and K9 have a common terminal and have their other terminals connected to input terminals a and b, respectively, to form a second pair. A single inhibitor tube 19 has one terminal connected to the common terminal, and its other terminal connected to output terminal d through a resistor R9 equal in value to resistors R5, R7 and R8. A supply resistor RD2, which may be equal in value to resistor RD1, is connected between the common terminal of indicator tubes K8 and K9 and output terminal c of unit UD, which in turn is connected to output terminal of blanking unit 4.

In operation, as stated above, unit UD functions in a manner similar to unit UA described above. Each of the indicator tubes may be ionized by applying ground potential to the counter terminal d or e corresponding to its input terminal if none of the inhibitor tubes con nected to its other terminal is conducting. The possible combinations are shown in Table II below. The 0s and 1s in Table II refer to ground and N /2 potentials as applied to the terminals of unit UD except that the Os differ from ground potential by the amount determined by the current through an ionized indicator tube for terminals a and b. As before, the 0s and ls applied to the tubes K6-K9 and 16-19 indicate extinguished and ionized tubes, respectively.

Table II UD Terminals UD Output Count 11 b I d e f 9 K6 K7 1G 17 K8 K9 18 19 1 O 1 0 0 0 0 1 1 O 0 0 1 0 0 1 0 O 0 O 1 1 0 0 0 1 1 0 1 0 1 0 O 0 1 0 0 0 1 0 0 1 O 1 0 0 0 1 0 0 0 1 1 0 1 1 0 0 0 1 0 0 0 0 1 0 0 1 l 0 0 1 0 O O 0 0 1 1 0 1 1 1 1 O 0 0 0 O 0 1 0 O 1 1 1 0 1 0 0 0 0 0 1 1 1 0 1 1 0 0 0 0 1 0 1 0 0 1 0 1 l 0 0 0 0 0 1 1 0 In Table II, it will be noted that at counts 0 and 1, inhibitor tubes 16 and 17 are both required to be ionized. As explained above, depending on the values of the components and of the supply voltage, either or both of these tubes may be ionized, but as long as one tube is ionized the operation of the indicator tubes will not be altected.

The overall operation of the illustrated embodiment of my invention is summarized in FIG. 4. At each count, a corresponding condition of the terminals of the various units are shown by Os indicating ground potential and 1s indicating a potential of N1/2, except as applied to terminals a and b of the units UA-UD as explained above. The corresponding states of the indicator and inhibitor tubes are shown in columns designated by the reference characters of the tubes. Here, as 0 indicates an extinguished tube, and a 1 a conducting tube. It will be noted that one and only one indicator tube is conducting at each count, the conducting indicator tube being indicated in FIG. 4 by a heavy line around the entry in the table so that the count progression can be visualized at a glance.

The operation of the system shown in FIG. 1 will be apparent from the above description. Briefly, as each pulse is supplied from source 1, counter 2 responds by a change of state of its output terminals 0!, e, f, g, h, i, j and k in the manner shown in FIG. 4. As the count progresses, the digital indicator 3 is energized to cycle its indicating tubes in the manner shown in FIG. 4. In any state of the indicator, the output terminal a or b connected to the conducting indicator tube, and to the corresponding indicator tubes in the other units, will be below ground potential as explained above, such that the voltage across the other indicator tubes are reduced to positively prevent improper operation. Should the repetition rate of the pulse become too great to visually interpret, or if it is desired to blank the unit during counting, blanking unit 4 will function to reduce the supply voltage to indicator 3 and extinguish the tubes. It should be noted in this regard that the operation of the indicator of my invention is completely independent of the counter, and blanking of the indicator does not interfere with the progress of the count. Moreover, since the indication is positively determined at each count and does not depend on the previous count for proper operation, the indicator will respond properly when reenergized at any point in the cycle.

While I have shown only a single decade in the system of FIG. 1, it will be apparent that any number of decades could be cascaded. The input to the next decade from the system of FIG. 1 would be supplied by output terminal k of counter 2, at which a positive-going pulse will appear every tenth count which would be suitable for application to the input terminal a of a succeeding counter 2.

I have illustrated my invention with emphasis on the advantages to be obtained with neon glow-tubes. However, other bistable devices exhibiting two discrete impedance states may also be employed, if so desired, without departing from the scope of my invention.

The values of the components employed in the indicator and counting system of my invention can obviously be selected from a wide range of values in accordance with conventional design practice, in a manner that will be obvious to those skilled in the art. However, as a specific example, I have found that one set of suitable values for the components are as shown in Table III below.

In Table III, values of resistors are given on ohms, and all resistors are /2 watt with tolerances of plus or minus 5 percent unless otherwise noted. The values of the capacitors are in microfarads, and these should also have 5 percent tolerances.

Table III Component: Value RER9 82K RIG-R12 6.8K R13, R14, R27 22K R15, R17 56K, lW R16, RIS-RZII K R21 240K R22 20K, 5W R23 3.3K, 1W R24 62K R25, R26 56K RARC, RD1, RDZ, RE, RF 220K C1-C3 120 C4 68 C5 In reading Table III above, it should be understood that the designation of the components of units UB and UC follow the scheme sufliciently illustrated. in units UA and UD. That is, the supply resistors of the units have a suffix B for unit UB and C for unit UC, and the inhibitor tube supply resistors have suflixes corresponding to the sufiix of the associated inhibitor tubes. All of the diodes shown in the circuits may be of the type 1N636, and all transistors may be of the type 2N398. All of the neon glow-tubes may be of the NE-2 type. Finally, comparing Table III with FIG. 2, in bistable unit F4 the capacitors corresponding to C4 and C5 in that unit were given values of 220 micromicrofarads and 120 micromicrofarads, respectively.

While I have described only one embodiment of my invention in detail, many changes and variations will become apparent to those skilled in the art upon reading my description, and such changes can obviously be made without departing from the scope of my invention.

Having thus described my invention, what I claim is:

1. A logic circuit, comprising, in combination, first, second, third and fourth terminals, a first resistor connected between said first and fourth terminals, a first neon glowtube connected in series between said second and said fourth terminals, and a second resistor and a second neon glow-tube connected between said third and said fourth terminals, whereby said second tube conducts and said first tube does not conduct when first and second voltages of predetermined amounts are applied between said first and second terminals and between said first and third terminals, respectively, said first voltage being smaller than said second voltage and sufficient to cause conduction of said first tube in the absence of said second voltage.

2. A logic circuit, comprising first, second, third and fourth terminals, a first resistor connected between said first and fourth terminals, a first neon glow-tube and a second resistor connected in series between said first and second terminals, and a third resistor and a second neon glow-tube connected between said first and third terminals, said second resistor being larger than said third resistor, whereby said second tube conducts and said first tube does not conduct when first and second equal voltages of predetermined magnitude are applied between said second and fourth and said third and fourth terminals.

3. In a logic circuit, first and second bistable devices having a common terminal and each having a free terminal, each device exhibiting a first or a second impedance state according as a voltage below or above a predeter mined value is applied across its terminals, a first impedance having a free terminal connected to said common terminal, second and third impedances connected in series be- .tween said free terminals, first and second independent current supply means connected across said second and third impedances, respectively, each current supply means being operable when actuated to supply current through its associated resistor, and means for applying a voltage between the free terminal of said first impedance and the junction of second and third impedances to cause said first, said second, or neither of said bistable devices to change from its first to its second impedance state according as said first, said second, or both said current supply means are actuated.

4. In combination, an array of neon glow-tubes, an energizing circuit for said glow tubes comprising a common impedance and an individual impedance for each glowtube, said individual impedances having a common terminal, means for applying a voltage between said common terminal and said common impedance, and means for in hibitin-g the ionization of all but one of said glow-tubes comprising a bistable current source connected across each individual impedance, each source having a first state in which no current is supp-lied to said impedance and a second state in which current is supplied-to said impedance in an amount sufiicient to prevent ionization of the associated tube, andmeans for actuating all but one of said sources from its first to its second state.

'5. A logic circuit, comprising, first, second, third and fourth terminals, a first resistor, a first neon glow-tube, a second neon glow-tube, and a second resistor connected in that order in series between said first terminal and said second terminal, a third resistor connected between the junction of said first and second tubes and said third terminal, a third neon glow-tube, a fourth neon glowtube, and a fourth resistor connected in that order in series between the junction of said first resistor and said first tube and said fourth terminal, and a fifth resistor connected between the junction of said third and fourth tubes and said third terminal, the value of the first resistor being larger than the value of the second and fourth resistors, whereby upon the application of a voltage between the first and third terminals sufficient to ionize a tube, the first or the third tube will be ionized as the same voltage is applied across the second and third or the fourth and third terminals, respectively.

. 6. A logic circuit comprising first, second, third and fourth terminals, first and second two-terminal bistable devices each of which exhibits one of two discrete conductive states according respectively as one of two discrete voltage states is established across the terminals of said device, said devices having a terminal of each connected in common to said first terminal, a first resistive impedance connected between said first and said fourth terminals, the other terminal of said first of said devices being connected through a second resistive impedance to said second terminal, the other terminal of said second of said devices being connected through a third resistive impedance to said third terminal, the resistivity of said second impedance being greater than the resistivity of said third impedance.

7. A logic circuit as defined in claim 6, wherein each of said devices is a gaseous discharge tube.

8. In combination, an array of two-terminal bistable devices each of which is capable of exhibiting two discrete, mutually exclusive, impedance states according respectively to one of two discrete voltage states established across its terminals, each of said devices having one terminal thereof connected through a common resistive impedance to a first common terminal, and having the other terminal thereof respectively connected through individual associated resistive impedances to a second common terminal, means for applying a voltage between said first and second common terminals, means for inhibiting the more conductive of said impedance states of all but one of said devices, said inhibiting means comprising in-- dividual bistable current sources respectively connected across each of said individual impedances, each source having a first state in which said source is adapted not to supply current to its associated individual impedance and a second state in which said source is adapted to supply current to said associated individual impedance in an amount sufiicient to prevent the associated device from assuming its more conductive state, and means for actuating all but one of said sources from its first to its second state.

9. A combination as defined in claim 8 wherein each of said bistable devices is a gaseous discharge device.

10. A combination as defined in claim 8 wherein each of said bistable current sources comprises a gaseous discharge tube and an associated current supplying resistor.

11. An indicator logic circuit comprising in combination, a predetermined number n of two-terminal gaseous glow-tubes in a first array, means for energizing said glow-tubes including a common resistance connected to all of one of the terminals of each of said glow-tubes and a plurality of individual resistors each connected in parallel to one another to a common terminal and in series with a corresponding other of said terminals of said glowtubes, means for applying a voltage between said common resistance and said common terminal, and a number 11-1 of inhibiting two-terminal gaseous glow-tubes in a second array wherein each of said inhibiting tubes is coupled at one terminal to said other terminal of a corresponding glow-tube of said first array and is connected through a corresponding input resistor to source of a binary signal, said common resistance being greater in magnitude than any of said input resistors.

No references cited.

MAYNARD R. WILBUR, Primary Examiner. J. F. MILLER, Assistant Examiner. 

1. A LOGIC CIRCUIT, COMPRISING, IN COMBINATION, FIRST SECONE, THIRD AND FOURTH TERMINALS, A FIRST RESISTOR CONNECTED BETWEEN SAID FIRST AND FOURTH TERMINALS, A FIRST NEON GLOW TUBE CONNECTED IN SERIES BETWEEN SAID THIRD AND SAID FOURTH FOURTH TERMINALS, AND A SECOND RESISTOR AND A SECOND NEON GLOW-TUBE CONNECTED BETWEEN SAID THIRD AND SAID FOURTH TERMINALS, WHEREBY SAID SECOND TUBE CONDUCTS AND SAID FIRST TUBE DOES NOT CONDUCT WHEN FIRST AND SECOND VOLTAGES OF PREDETERMINED AMOUNTS ARE APPLIED BETWEEN SAID FIRST AND SECOND TERMINALS AND BETWEEN SAID FIRST AND THIRD TERMINALS, RESPECTIVELY, SAID FIRST VOLTAGE BEING SMALLER THAN SAID SECOND VOLTAGE AND SUFFICIENT TO CAUSE CONDUCTION OF SAID FIRST TUBE IN THE ABSENCE OF SAID SECOND VOLTAGE. 